In general, a user equipment (UE) or user apparatus measures quality of adjacent cells being handover target candidates and reports the measurement result to a base station in advance of handover (HO). The quality may be represented by reception level or reception SINR of a reference signal, for example. The reporting to the base station (eNB) is carried out through measurement reports. Based on the measurement report, the base station determines whether the user equipment UE is to initiate the handover and transmits a handover instruction message as a handover command to the user equipment UE.
The handover target may be not only a cell having the same frequency in the same system but also a cell having a different frequency in the same system. Alternatively, the handover target may be a cell where a different radio access technology (RAT) is employed. The frequency of the cell using such a different radio access technology may be typically different from that of the handover source. Thus, the frequency of the handover target cell may be necessarily different from that of the handover source cell.
FIG. 1 schematically illustrates exemplary handover between different frequency cells. In FIG. 1, a LTE (Long Term Evolution) based system including a mobile communication system having a first frequency f1 and a mobile communication system having a second frequency f2 and a WiMAX based system using a frequency f3 different from these frequencies are illustrated. Handover between different frequency systems or different RAT systems may be described in a version of 3GPP TS25.331 at filing the present application, for example.
Meanwhile, a user equipment (UE) typically has one radio frequency signal processing unit and accordingly cannot transmit or receive signals to/from the different frequency systems simultaneously. For this reason, in order to measure a cell (different frequency cell) having a frequency different from that of a camped cell (serving cell), the user equipment must perform frequency resynchronization. Specifically, the base station (eNB) indicates a length of a gap period, an arrival cycle of the gap period, a frequency of a different frequency cell or others to the user equipment (UE) through RRC measurement control. The user equipment (UE) conducts different frequency measurement, including change in frequencies, following synchronization channels, quality measurements or others, during the indicated gap period. The term “different frequency measurement” used herein includes not only searching for different frequency cells and measuring the quality but also searching for different RAT cells and measuring the quality.
FIG. 2 schematically illustrates exemplary gap periods in communications at a serving cell.
In order to realize rapid handover to a different frequency cell, different frequency measurement must be efficiently and rapidly conducted ahead of the handover. For this reason, it is desired to specify an operating pattern of the user equipment during the gap period as strictly as possible and optimize a specific manner of the different frequency measurement. Such an operating pattern highly depends on the length of the gap period in the camped cell and transmission frequencies of synchronization channels, pilot signals, broadcast information and others in the handover target. Thus, in order to optimize the different frequency measurement method, it is desirable to predetermine how the user equipment needs to operate corresponding to envisaged different situations depending on the length of the gap period in the camped cell and transmission frequencies of synchronization channels, pilot signals, broadcast information and others in the handover target. If it can be predetermined how the user equipment needs to operate in this manner, the different frequency measurement can be rapidly completed.
Meanwhile, handover target systems typically possess their own specific transmission periodicities of the synchronization channels, the pilot signals and the broadcast information. This means that the handover target systems may have different optimum lengths of the gap period. In other words, a camped cell may need kinds of different gap period corresponding to the number of potential handover target systems. In this case, however, the user equipment might have to optimize the different frequency measurement corresponding to the kinds of gap periods, resulting in complication of the user equipment and higher design costs. Furthermore, it would have to be guaranteed that the user equipment operates to conduct the optimum different frequency measurements for all the gap periods, resulting in heavier workloads on operation tests for the user equipment.
On the other hand, although it is desirable to apply a single type of gap period for any handover target system, it is hard to define the optimum single gap period for existing radio communication systems and future radio communication systems.